The present invention relates to a gas sensor element for detecting the concentration of a specific gas component in gas under measurement such as combustion gas or exhaust gas of an internal combustion engine or a combustor etc. and to a gas sensor using the gas sensor element.
Hereinafter, the terms “front” and “rear” are used with respect to the axial direction of a gas sensor (gas sensor element) for purposes of description. These terms are illustrative and are not intended to limit the scope of the present invention.
There has been used a gas sensor having a gas sensor element for detecting the concentration of a specific gas component such as oxygen in exhaust gas of an internal combustion engine. The gas sensor element includes a plate-shaped element body having, at a front end portion thereof, a gas sensing portion provided with a solid electrolyte substrate and a pair of electrodes. When the front end portion of the gas sensor element in which the gas sensing portion of the element body is located (also referred to as “the sensing end portion of the gas sensor element”) is exposed to the exhaust gas, poisoning substances such as silicon and phosphorus in the exhaust gas may be adhered to the sensing end portion of the gas sensor element. Water content such as condensed water in the exhaust gas or in an exhaust pipe of the internal combustion engine may also be adhered to the sensing end portion of the gas sensor element. At least the sensing end portion of the gas sensor element is thus covered with a porous ceramic protection layer so as to trap poisoning substances and prevent direct contact of water content with the sensing end portion of the gas sensor element. Japanese Laid-Open Patent Publication No. 2003-322632 discloses one such type of porous protection layer having a two-layer structure in which an inner (lower) layer is higher in porosity than an outer (upper) layer. In this protection layer, the inner layer has roughness due to its high porosity and thereby exhibits anchoring effect so as to improve the adhesion of the inner layer to the outer layer. The inner layer also exhibits thermal insulation effects due its high porosity so as to, even when the gas sensor element gets wet with water (water drop becomes adhered to the porous protection layer), prevent heat from being taken away from the gas sensing portion to the outer layer.